The diagnosis of malignant cells in body fluids and fine needle aspirates can be extremely difficult using morphological criteria alone. Using lymphoid cells surface markers we have developed a technique for the immunocytochemical diagnosis of lymphoma in cytological specimens. This methodology is applicable to all types of malignant disease, provided one has suitable cell surface markers. We have investigated various markers for their usefulness in the diagnosis of metastatic carcinoma in body cavity fluids and needle aspirates. This work should greatly enhance the power of cytological diagnosis.